Phase shift mask blank, phase shift mask, and method of manufacture

ABSTRACT

In a phase shift mask blank comprising a transparent substrate and a phase shift film thereon, after the phase shift film is formed on the substrate, it is surface treated with ozone water having an ozone concentration of at least 1 ppm. The resulting phase shift film is of quality in that it experiences minimized changes of phase difference and transmittance upon immersion in chemical liquid during subsequent mask cleaning step or the like.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to phase shift mask blanks andphase shift masks suitable for use in the microfabrication of electronicproducts such as semiconductor integrated circuits. It relates also tomethods of manufacturing such phase shift mask blanks and phase shiftmasks. More particularly, it relates to halftone phase shift mask blanksand phase shift masks which can attenuate the intensity of exposurewavelength light with a phase shift film, and to methods ofmanufacturing such phase shift mask blanks and phase shift masks.

[0003] 2. Prior Art

[0004] Photomasks are used in a broad range of applications, includingthe manufacture of semiconductor integrated circuit (IC), large-scaleintegration (LSI) and VLSI chips. They are basically constructed bystarting with a photomask blank comprising a transparent substrate and alight-shielding film made primarily of chromium thereon and processingthe light-shielding film by photolithography using UV radiation orelectron beams for thereby forming a desired pattern in the film. Themarket demand for ever higher levels of integration in semiconductorintegrated circuits has led to a rapid reduction in the minimum featuresize of photomask patterns. Such miniaturization has been achieved inpart by the use of shorter wavelength exposure light.

[0005] Although exposure using shorter wavelength light does improveresolution, it has undesirable effects, such as reducing the focaldepth, lowering process stability and adversely impacting product yield.

[0006] One pattern transfer technique that has been effective forresolving such problems is phase shifting. This involves the use of aphase shift mask as the mask for transferring microscopic circuitpatterns.

[0007] As shown in accompanying FIGS. 7A and 7B, a phase shift mask(typically, halftone phase shift mask) is generally composed of asubstrate 1 on which a phase shift film 2 has been patterned. The maskhas both exposed substrate areas (first light-transmitting areas) 1 a onwhich there is no phase shift film, and phase shifters (secondlight-transmitting areas) 2 a that form a pattern region on the mask.The phase shift mask improves the contrast of a transferred image byproviding, as shown in FIG. 7B, a phase difference of 180 degreesbetween light passing through the pattern region and light passingthrough the non-pattern region, and utilizing the destructiveinterference of light at the boundary regions of the pattern to set thelight intensity in the areas of interference to zero. The use of phaseshifting also makes it possible to increase the focal depth at thenecessary resolution. Hence, compared with a conventional mask having anordinary light-shielding pattern such as chromium film, the phase shiftmask can improve resolution and increase the margin of the exposureprocess.

[0008] For practical purposes, such phase shift masks can be broadlycategorized, according to the light-transmitting characteristics of thephase shifter, as either completely transmitting phase shift masks orhalftone phase shift masks. Completely transmitting phase shift masksare masks in which the phase shifter has the same light transmittance asthe substrate, and which are thus transparent to light at the exposurewavelength. In halftone phase shift masks, the phase shifter has a lighttransmittance that ranges from about several percent to several tens ofpercent the transmittance of exposed substrate areas.

[0009] Accompanying FIG. 1 shows the basic structure of a halftone phaseshift mask blank, and FIG. 2 shows the basic structure of a halftonephase shift mask. The halftone phase shift mask blank shown in FIG. 1includes a transparent substrate 1 and a halftone phase shift film 2formed over the substantially entire surface of the substrate 1. Thehalftone phase shift mask shown in FIG. 2 is arrived at by patterningthe phase shift film 2 of the blank and includes phase shifters 2 awhich form the pattern regions of the mask and exposed substrate areas 1a on which there is no phase shift film. Exposure light that has passedthrough the phase shifter 2 a is phase-shifted relative to exposurelight that has passed through the exposed substrate area 1 a (see FIGS.7A and 7B). The transmittance of the phase shifter 2 a is selected suchthat exposure light which has passed through the phase shifter 2 a hastoo low an intensity to sensitize the resist on the substrate to whichthe pattern is being transferred. Accordingly, the phase shifter 2 afunctions to substantially shield out the exposure light.

[0010] Halftone phase shift masks of the above type encompass halftonephase shift masks of the single-layer type which are simple in structureand easy to manufacture. Single-layer halftone phase shift masks knownto the art include those described in JP-A 7-140635 which have a phaseshifting film composed of a molybdenum silicide material such as MoSiOor MoSiON.

[0011] One important feature for these phase shift masks and phase shiftmask blanks is resistance to acids, for example, chemical liquids suchas sulfuric acid and aqueous persulfuric acid (mixture of sulfuric acidand aqueous hydrogen peroxide) used in the resist removing and cleaningsteps of the mask manufacture process, and chromium etchants having ahigh oxidizing power used in removal of chromium film.

[0012] Prior art phase shift films are less resistant to chemicalliquids and raise a problem that the cleaning or chromium etching stepresults in deviations of phase difference and transmittance from thepreset values.

SUMMARY OF THE INVENTION

[0013] It is therefore an object of the invention to provide phase shiftmask blanks and phase shift masks of quality which experience littlechanges of phase difference and transmittance under the action ofchemical liquids used for cleaning. Another object of the invention isto provide methods of manufacturing such phase shift mask blanks andphase shift masks.

[0014] The invention addresses a phase shift mask blank comprising asubstrate which is transparent to exposure wavelength and at least onelayer of phase shift film thereon. It has been found that once the phaseshift film is formed, it is surface treated with ozone water having atleast 1 ppm of ozone dissolved therein whereby the surface of the phaseshift film is modified such that the phase shift film will experiencelittle changes of phase difference and transmittance when later treatedwith chemical liquids in cleaning and chromium etching steps. Theresulting phase shift mask blank is of high quality in this sense, and aphase shift mask obtained therefrom is as well.

[0015] In a first aspect, the present invention provides a phase shiftmask blank comprising a transparent substrate and at least one layer ofphase shift film thereon. In a first embodiment, the phase shift filmhas been surface treated with ozone water having at least 1 ppm of ozonedissolved therein. In a second embodiment, the phase shift film isformed of a metal silicide oxide, metal silicide nitride or metalsilicide oxynitride and then surface treated with ozone water having atleast 1 ppm of ozone dissolved therein. In a third embodiment, the phaseshift film is formed of molybdenum silicide oxide, molybdenum silicidenitride or molybdenum silicide oxynitride and then surface treated withozone water having at least 1 ppm of ozone dissolved therein.

[0016] Preferably, the phase shift film has an oxygen content and asilicon content at its surface, and a molar ratio of the oxygen contentto the silicon content is at least 1. Also preferably, the phase shiftfilm changes the phase of exposure light passing therethrough by 180±5degrees and has a transmittance of 3 to 40%.

[0017] In a second aspect, the present invention provides a phase shiftmask obtained by patterning the phase shift film in the inventive phaseshift mask blank.

[0018] In a third aspect, the present invention provides a method ofmanufacturing a phase shift mask blank comprising a transparentsubstrate and at least one layer of phase shift film thereon. In a firstembodiment, the method includes the steps of forming the phase shiftfilm on the substrate and surface treating the phase shift film withozone water having at least 1 ppm of ozone dissolved therein. In asecond embodiment, the method includes the steps of forming the phaseshift film of a metal silicide oxide, metal silicide nitride or metalsilicide oxynitride on the substrate and surface treating the phaseshift film with ozone water having at least 1 ppm of ozone dissolvedtherein. In a third embodiment, the method includes the steps of formingthe phase shift film of molybdenum silicide oxide, molybdenum silicidenitride or molybdenum silicide oxynitride on the substrate and surfacetreating the phase shift film with ozone water having at least 1 ppm ofozone dissolved therein.

[0019] In a fourth aspect, the present invention provides a method ofmanufacturing a phase shift mask, comprising the steps of forming byphotolithography a patterned resist film on the phase shift film in thephase shift mask blank obtained by the above method, etching away theportions of the phase shift mask which are uncovered with the resistfilm, and thereafter, removing the resist film.

[0020] According to the invention, once the phase shift film is formed,it is surface treated with ozone water having at least 1 ppm of ozonedissolved therein whereby the outermost surface of the film is uniformlyoxidized such that the phase shift film will experience little changesof phase difference and transmittance when later treated with chemicalliquids such as aqueous persulfuric acid in the mask manufacturingprocess.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The objects, features and advantages of the invention will becomemore apparent from the following detailed description, taken inconjunction with the accompanying drawings.

[0022]FIG. 1 is a sectional view of a phase shift mask blank accordingto one embodiment of the invention.

[0023]FIG. 2 is a sectional view of a phase shift mask made from thesame blank.

[0024]FIG. 3 is a sectional view of a phase shift mask blank having a Crbase light-shielding film according to another embodiment of theinvention.

[0025]FIG. 4 is a sectional view of a phase shift mask blank having a Crbase light-shielding film and a Cr base antireflection film according toa further embodiment of the invention.

[0026]FIG. 5 is a sectional view of a phase shift mask blank accordingto a still further embodiment of the invention.

[0027]FIGS. 6A to 6D are sectional views illustrating the method ofmanufacturing a phase shift mask from a phase shift mask blank accordingto the invention. FIG. 6A shows a mask blank on which a resist film hasbeen formed, FIG. 6B shows the structure after the resist film has beenpatterned, FIG. 6C shows the structure after etching has been carriedout, and FIG. 6D shows the completed mask after the resist film has beenremoved.

[0028]FIGS. 7A and 7B illustrate the operating principle of a halftonephase shift mask. FIG. 7B is an enlarged view of region X in FIG. 7A.

[0029]FIG. 8 is a diagram showing the composition in depth direction ofa phase shift film as analyzed by ESCA.

DETAILED DESCRIPTION OF THE INVENTION

[0030] Referring to FIG. 1, a phase shift mask blank according to theinvention includes a substrate 1 of quartz, CaF₂ or the like which istransparent to exposure light and at least one layer of phase shift film2. After the phase shift film 2 is formed on the substrate 1, it issurface treated with ozone water having at least 1 ppm of ozonedissolved therein. The phase shift mask of the invention is obtained bypatterning the phase shift film 2 of the phase shift mask blank shown inFIG. 1. The mask has patterned phase shifters 2 as shown in FIG. 2,wherein first light-transmitting areas (exposed substrate areas) 1 a aredefined between the patterned phase shifters and secondlight-transmitting areas 2 a are provided by the patterned phaseshifters.

[0031] The phase shift film is preferably formed of a metal silicideoxide, metal silicide nitride or metal silicide oxynitride, and morepreferably molybdenum silicide oxide (MoSiO), molybdenum silicidenitride (MoSiN) or molybdenum silicide oxynitride (MoSiON).

[0032] For depositing the phase shift film, a reactive sputteringtechnique is preferred. When a metal silicide oxide, metal silicidenitride or metal silicide oxynitride is to be deposited, the target usedin sputtering is a metal silicide target containing that metal. Whenmolybdenum silicide oxide, molybdenum silicide nitride or molybdenumsilicide oxynitride is to be deposited, a molybdenum silicide target isused. To maintain a constant film composition, a metal silicide havingeither one or both of oxygen and nitrogen added thereto may be used.

[0033] The sputtering process may employ a direct-current power supply(DC sputtering) or a high-frequency power supply (RF sputtering). Eithera magnetron sputtering system or a conventional sputtering system may beused. The film-forming system may be either a continuous, in-line systemor a single-wafer processing system.

[0034] The sputtering gas may contain an inert gas such as argon orxenon, nitrogen gas, oxygen gas and various nitrogen oxide gases, whichare suitably combined so that the phase shift film deposited therefrommay have a desired composition.

[0035] Where it is desired to increase the transmittance of a phaseshift film to be deposited, it is recommended that the amounts of oxygenand nitrogen-containing gases in the sputtering gas be increased so thatmore oxygen and nitrogen are taken into the film, or a metal silicidehaving more oxygen and nitrogen previously added thereto be used as thesputtering target.

[0036] In the phase shift mask blank of the invention, the phase shiftfilm may include a plurality of layers.

[0037] According to the invention, after the phase shift film is formedon the substrate, it is surface treated with ozone water. The ozonewater used herein should preferably have an ozone concentration of atleast 1 ppm, more preferably at least 5 ppm. The upper limit of theozone concentration is not critical and may be the ozone saturationlevel. Specifically, the ozone concentration is often up to 50 ppm, andespecially up to 20 ppm. The ozone treatment may be carried out byimmersing the phase shift mask blank in a tank full of ozone water, orby flowing ozone water over the phase shift mask blank while spinningthe blank. The treatment temperature may be in the range of 0 to 60° C.,especially 0 to 30° C. although room temperature is most often used. Thetreatment time may be suitably determined and is preferably about 1 to10 minutes, especially about 2 to 5 minutes.

[0038] preferably, the phase shift film has an oxygen content and asilicon content at its surface, and a molar ratio of the oxygen contentto the silicon content is at least 1 and more preferably between 1 and2.

[0039] By the action of ozone water, the outermost surface of the phaseshift film is oxidized and stabilized so that it will experience littlechanges of phase difference and transmittance when later contacted withchemical liquids such as sulfuric acid.

[0040] In summary, the phase shift mask blank of the invention includesa transparent substrate and at least one layer of phase shift film whichis deposited thereon, preferably by a reactive sputtering techniqueusing a sputtering gas containing oxygen and/or nitrogen and thentreated with ozone water containing at least 1 ppm of ozone, whichensures that the phase shift film has a transmittance of several percentto several tens of percent, especially 3 to 40% to exposure light andthe resulting phase shift mask provides a phase difference of 180°±5°between light transmitted by the phase shifter and light transmitted bythe transparent substrate.

[0041] In another embodiment, the phase shift mask blank of theinvention may include a chromium-base light-shielding film 3 which isformed on the phase shift film 2 (which has been surface treated withozone water) as shown in FIG. 3. In a further embodiment, as shown inFIG. 4, a chromium-base antireflection film 4 may be formed on thechromium-base light-shielding film 3 for reducing reflection from thechromium-base light-shielding film 3. In a still further embodiment, asshown in FIG. 5, a phase shift film 2, a first Cr base antireflectionfilm 4, a Cr base light-shielding film 3, and a second Cr baseantireflection film 4′ are formed on the substrate 1 in order.

[0042] By patterning the phase shift film of the phase shift mask blankaccording to the invention, a phase shift mask as shown in FIG. 2 ismanufactured. A typical process is shown in FIG. 6. After a phase shiftfilm 12 is formed on a transparent substrate 11 and surface treated withozone water containing at least 1 ppm of ozone, a resist film 13 isformed on the phase shift film 12 (FIG. 6A). Next, the resist film 13 islithographically patterned (FIG. 6B), after which the phase shift film12 is etched (FIG. 6C). The resist film 13 is subsequently stripped(FIG. 6D). In this process, application of the resist film, patterning(exposure and development), etching and resist film removal may becarried out by known techniques.

EXAMPLE

[0043] Examples and comparative examples are given below by way ofillustration, and are not intended to limit the scope of the invention.

Example 1

[0044] A molybdenum silicide oxynitride (MoSiON) film of 120 nm thickwas deposited on a quartz substrate of 6 inches square by a DCsputtering technique using molybdenum silicide as the sputtering targetand a gas mixture of argon, nitrogen and oxygen in a flow ratio of1:15:1 as the sputtering gas at a discharge power of 200 W, and adeposition temperature of 200° C. The phase shift film as deposited hada phase difference of 185.4° and a transmittance of 5.62%.

[0045] Using a spin washer, the sample was treated for 5 minutes withozone water having an ozone concentration of 18 ppm at room temperatureand then spin dried. The ozone-treated phase shift film had a phasedifference of 184.6° and a transmittance of 5.82%.

[0046] The sample was immersed in a chemical liquid which was a 1:4mixture of sulfuric acid and aqueous hydrogen peroxide at 80° C. for 2hours. The sample was measured for phase difference and transmittanceagain, and its chemical resistance was evaluated from changes of phasedifference and transmittance before and after the chemical immersion.

[0047] The sample after 2 hours of immersion in chemical liquid had aphase difference of 182.1° and a transmittance of 6.50%. By the chemicalimmersion, the phase difference changed −2.5° and the transmittancechanged +0.68%. The results are shown in Table 1.

[0048] It is noted that the phase difference and transmittance weremeasured by MPM-248 by Laser Tec Co. at a wavelength of 248 nm.

[0049] The composition of the sample after ozone water treatment wasanalyzed in a depth direction by ESCA. The results are shown in FIG. 8,which indicates that the oxygen content within the film has a molarratio of about 0.3 to the silicon content whereas the oxygen content atthe outermost surface has a molar ratio of 1.2 to the silicon content.

Comparative Example 1

[0050] A molybdenum silicide oxynitride (MoSiON) film of 118 nm thickwas deposited as in Example 1. The phase shift film as deposited had aphase difference of 178.8° and a transmittance of 6.09%.

[0051] The sample was tested for chemical resistance as in Example 1.The sample after 2 hours of immersion in chemical liquid had a phasedifference of 173.9° and a transmittance of 7.61%. By the chemicalimmersion, the phase difference changed −4.9° and the transmittancechanged +1.52%. The results are also shown in Table 1.

[0052] It is noted that the measurement of phase difference andtransmittance is the same as in Example 1. TABLE 1 Chemical resistanceBefore After immersion immersion Change Example 1 Phase difference184.6° 182.1° −2.5° Transmittance 5.82% 6.50% +0.68% Comparative Phasedifference 178.8° 173.9° −4.9° Example 1 Transmittance 6.09% 7.61%+1.52%

[0053] It is evident that the ozone water treatment of the phase shiftfilm after its formation improves the acid resistance of the film sothat the resulting phase shift mask blank (and phase shift mask thereof)is of quality in that it experiences minimized changes of phasedifference and transmittance upon contact with chemical liquid duringsubsequent cleaning step.

[0054] Although ozone water having an ozone concentration of 18 ppm wasused in Example, the invention is not limited thereto. Treatment withozone water is effective to the purpose of the invention as long asozone is contained even in a minute amount, specifically at least 1 ppm.

[0055] Although a molybdenum silicide oxynitride film was used inExample, the invention is not limited thereto. Equivalent results areobtainable on use of molybdenum silicide oxide or molybdenum silicidenitride. This is also true to phase shift mask blanks and phase shiftmasks using silicide oxides, silicide nitrides or silicide oxynitridesof metals other than molybdenum as the phase shift film. Moreover, theinvention is not limited to those containing phase shift mask blanks andphase shift masks having phase shift films of metal silicides, andequivalent results are obtainable from phase shift mask blanks and phaseshift masks having other phase shift films.

[0056] There have been described a phase shift mask blank and a phaseshift mask thereof in which a phase shift film is formed by a reactivesputtering technique and then treated with ozone water whereby theresulting phase shift film is of quality in that it will experienceminimized changes of phase difference and transmittance upon contactwith chemical liquid during subsequent mask cleaning step or the like.

[0057] Japanese Patent Application No. 2001-237670 is incorporatedherein by reference.

[0058] Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A phase shift mask blank comprising a transparent substrate and atleast one layer of phase shift film thereon, wherein said phase shiftfilm has been surface treated with ozone water having at least 1 ppm ofozone dissolved therein.
 2. A phase shift mask blank comprising atransparent substrate and at least one layer of phase shift filmthereon, wherein said phase shift film is formed of a metal silicideoxide, metal silicide nitride or metal silicide oxynitride and thensurface treated with ozone water having at least 1 ppm of ozonedissolved therein.
 3. A phase shift mask blank comprising a transparentsubstrate and at least one layer of phase shift film thereon, whereinsaid phase shift film is formed of molybdenum silicide oxide, molybdenumsilicide nitride or molybdenum silicide oxynitride and then surfacetreated with ozone water having at least 1 ppm of ozone dissolvedtherein.
 4. The phase shift mask blank of claim 2 wherein said phaseshift film has an oxygen content and a silicon content at its surface,and a molar ratio of the oxygen content to the silicon content is atleast
 1. 5. The phase shift mask blank of claim 1 wherein said phaseshift film changes the phase of exposure light passing therethrough by180±5 degrees and has a transmittance of 3 to 40%.
 6. A phase shift maskobtained by patterning the phase shift film in the phase shift maskblank of claim
 1. 7. A method of manufacturing a phase shift mask blankcomprising a transparent substrate and at least one layer of phase shiftfilm thereon, said method comprising the steps of forming the phaseshift film on the substrate and surface treating the phase shift filmwith ozone water having at least 1 ppm of ozone dissolved therein.
 8. Amethod of manufacturing a phase shift mask blank comprising atransparent substrate and at least one layer of phase shift filmthereon, said method comprising the steps of forming the phase shiftfilm of a metal silicide oxide, metal silicide nitride or metal silicideoxynitride on the substrate and surface treating the phase shift filmwith ozone water having at least 1 ppm of ozone dissolved therein.
 9. Amethod of manufacturing a phase shift mask blank comprising atransparent substrate and at least one layer of phase shift filmthereon, said method comprising the steps of forming the phase shiftfilm of molybdenum silicide oxide, molybdenum silicide nitride ormolybdenum silicide oxynitride on the substrate and surface treating thephase shift film with ozone water having at least 1 ppm of ozonedissolved therein.
 10. The method of manufacturing a phase shift maskblank of claim 7 wherein said phase shift film changes the phase ofexposure light passing therethrough by 180±5 degrees and has atransmittance of 3 to 40%.
 11. A method of manufacturing a phase shiftmask, comprising the steps of forming by photolithography a patternedresist film on the phase shift film in the phase shift mask blankobtained by the method of claim 7, etching away the portions of thephase shift mask which are uncovered with the resist film, andthereafter, removing the resist film.